This invention relates to a rotation transmission device for carrying out changeover between transmission and shutoff of a driving force in a drive line of, for example, a vehicle.
Generally, in a 4 WD vehicle in which the front and rear wheels are directly connected together, when the vehicle turns on a paved road, a so-called tight corner braking phenomenon occurs. In order to solve this problem, for changeover between connection and disconnection of power to the front and rear wheels of a 4 WD vehicle, the present applicant has already proposed a rotation transmission device in which a roller type two-way clutch and an electromagnetic clutch are combined (JP patent publication 11-129779).
In the prior art rotation transmission device, the roller type two-way clutch is mounted between an inner member connected to an input shaft and an outer member provided around the inner member. An electromagnetic clutch is provided at one axial end of the two-way clutch.
In this two-way clutch, a plurality of cam surfaces are formed on the outer periphery of the inner member, a cylindrical surface is formed on the inner periphery of the outer member, a retainer is mounted between the inner member and the outer member, and rollers are mounted in pockets formed in the retainer. The elastic force of a switch spring is imparted to the retainer to keep the rollers in a neutral position in which the rollers are not in engagement with the cam surfaces of the inner member or the cylindrical surface of the outer member. When the inner member and the retainer rotate relative to each other against the elastic force of the switch spring, the rollers engage the cam surfaces of the inner member and the cylindrical surface of the outer member, thereby transmitting the rotation of the inner member to the outer member.
On the other hand, the electromagnetic clutch has a rotor mounted on the outer member so as to axially face an armature that is prevented from turning but that is axially movable relative to the retainer. An electromagnet is mounted in the rotor on the opposite side of the armature.
In this conventional rotation transmission device, when the electromagnetic coil of the electromagnet is not energized, the rollers are kept in the neutral position by the elastic force of the switch spring, so that the rotation of the inner member will not be transmitted to the outer member and the inner member idles relative to the outer member. On the other hand, when the electromagnetic coil of the electromagnet is energized, it will attract the armature to the rotor, so that the retainer will not rotate relative to the outer member. Due to the rotation of the inner member relative to the retainer, the rollers will engage the cam surfaces of the inner member and the cylindrical surface of the outer member, so that the rotation of the inner member is transmitted to the outer member through the rollers.
When the armature is attracted to the rotor, if the force for attracting the armature is weak, the armature will not be fixed to the outer member so as not to rotate relative to the outer member due to the elastic force of the switch spring. Therefore, it will be impossible to bring the engaging elements into engagement with the cam surfaces of the inner member and the cylindrical surface of the outer member. Thus it becomes necessary to impart a greater frictional torque to the armature so that the retainer will not return to the neutral position due to the turning torque by the switch spring.
In the conventional rotation transmission device, since the entire opposing surfaces of the rotor and the armature are attracting surfaces, the frictional torque when the rotor attracts the armature varies widely depending on the state of the attracting surfaces when in mutual contact, so that the frictional torque tends to be unstable. For example, if the rotor and the armature contact each other only at their inner-diameter side, the contact radius is small, so that the frictional torque. decreases correspondingly. Also, if the flatness of the attracting surfaces is poor, an air gap may develop. These portions become so-called air gaps through which magnetic fluxes are difficult to pass, thus lowering the attracting force. This makes it impossible to generate a predetermined frictional torque and delay response when the rollers engage. In the worst case, it may become impossible to cause the rollers to engage, so that no power transmitting state is obtained.
An object of this invention is to provide a rotation transmission device which makes it possible to stabilize the frictional torque when the rotor has attracted the armature.
According to this invention, there is provided a rotation transmission device comprising an inner member and an outer member coaxially mounted so as to be rotatable relative to each other, a retainer mounted between the inner member and the outer member and formed with a plurality of pockets, engaging elements mounted in the pockets, an armature mounted so as to be nonrotatable but axially movable relative to the retainer, a rotor mounted to one of the inner member and the outer member so as to axially oppose the armature, and an electromagnet for attracting the armature to the rotor. The phase of the retainer is changed to bring the engaging elements into engagement with the outer periphery of the inner member and the inner periphery of the outer member, and a turning torque is transmitted between the inner member and the outer member. When the armature is attracted to the rotor, the large-diameter portion of the armature is attracted to the rotor.
The means for attracting the large-diameter portion of the armature to the rotor may be a step formed on at least one of the opposed surfaces of the rotor and the armature so as to make the large-diameter portion higher than the small-diameter portion, or a tapered surface formed on at least one of the opposed surfaces of the rotor and the armature such that its axial height gradually lowers from the large-diameter side toward the small-diameter side.
With this arrangement, when the electromagnet is energized, the large-diameter portion of the armature is attracted to the rotor, so that the radius of the contact portion between the rotor and the armature stabilizes. This makes it possible to stabilize the frictional torque.
By forming an annular groove in the surface of the rotor that opposes the armature and forming arcuate elongated holes in the bottom of the annular groove in the circumferential direction, it is possible to prevent magnetic flux from directly passing through the attracting plate portion of the rotor so as to attract the armature more reliably.